Young adulthood (ages 18-24) is a critical developmental transition and provides an enormous opportunity to alter trajectories of smoking behavior for a large public health impact. Young adult smokers, in particular, have eluded both youth prevention and adult cessation intervention efforts. Recent increases in the rate of young adult smoking initiation and rate of transition to regular smoking have been reported along with the high prevalence of the phenomenon of social smoking in this age group (51%-62%). The long-term tobacco use behaviors of social smoking young adults (SSYAs) are unknown, but it is likely that approximately half will escalate to heavier cigarette use during young adulthood or that their pattern of social smoking will remain stable throughout adulthood. SSYAs present both a vital challenge and an opportunity for smoking cessation, but this requires a better understanding of potentially modifiable factors that contribute to risk. The current project uses primary socialization theory (PST) to address the intersection of social development and smoking behavior in young adulthood, identifying characteristics of and risk pathways leading from social smoking, a highly prevalent pattern of smoking among young adults. PST posits that individuals learn normative and deviant behavior mainly from a small number of social influences that change dynamically with lifespan transitions. The primary socialization influences in young adulthood are environmental (work, school), peer clusters, extended family, and the new family contexts created in this developmental period. The goal of the current study is to examine, in-depth, the characteristics of SSYAs, their potential smoking risk trajectories, and possible avenues for smoking cessation intervention. The proposal leverages information from two large contemporary cohorts of U.S. young adults (aged 18-24) with rich data on trajectories of cigarette smoking behavior, social influences on smoking, and social and contextual influences on in the moment smoking behavior among young adults. The two cohorts are: 1) a national sample of young adults aged 18-24 (n = 864 at baseline) Legacy Young Adult Cohort Study with semi- annual assessments over three years and 2) an at-risk group of adolescents followed during young adulthood (n = 1,027 at 5-year follow-up; NCI Program Project 2P01CA098262) using a combination of three annual assessments and two week-long sessions of daily ecological momentary assessment (EMA). Phase I of the study will use latent class analysis to define SSYAs in both cohorts (Aim 1) and Phase II will apply this definition to trajectory analyses in both cohorts, exploring tobacco use patterns among SSYAs compared to other young adult smokers (Aims 2 and 3). Phase III of the study takes a fine-grained approach to explore the proximal effect of social influences on smoking and cessation behavior and the potential of social smoking to serve as a moderator of this effect (Aim 4). This research lays the groundwork for developing more effective primary and secondary cancer prevention interventions for SSYAs by elucidating the social factors that maintain or impede smoking behavior in this important group.